Not all stressors are equal: Early social enrichment favors resilience to social but not physical stress in male mice

Early experiences profoundly affect the adult coping response to stress and, consequently, adult vulnerability to psychopathologies triggered by stressing conditions, such as major depression. Though studies in animal models have demonstrated that individuals reared in different conditions are differently vulnerable to a stressor of a specific quality, no information is available as to whether such vulnerability differs when facing stressors of different qualities. To this purpose, we reared C57BL/6 male mice either in standard laboratory rearing condition (SN) or in Communal Nest (CN) condition, the latter consisting of a single nest where three mothers keep their pups together and share care-giving behavior until weaning. We scored the amount of interactions with the mother and with peers and found that CN is a form of social enrichment because both these components are significantly increased. At adulthood, we exposed SN and CN mice, for 4 weeks, to either a physical (forced swim) or a social stress (social instability). Immediately before, at week 1 and at week 4 of the stress procedure, corticosterone levels and the hedonic profile were measured. The results show that CN mice are more resilient to social stress than SN mice since they displayed no anhedonia and lower corticosterone levels. By contrast, both experimental groups were similarly vulnerable to physical stress. Overall, our results show that, in male mice, the adult vulnerability to stress changes according to the quality of the stressor, as a function of early experiences. In addition, the stressor to which CN mice are resilient is qualitatively similar to the stimuli they have experienced early on, both concerning the social domain. (C) 2013 Elsevier Inc. All rights reserved.